A. Field of the Invention
This invention relates to the design of power supplies and more specifically to power supplies for corotrons to be used in duplicating and copying equipment.
B. Description of the Prior Art
A typical copier or duplicator contains a photoreceptor surface implemented from semi-conducting material formed onto a rotating drum. The drum surface will typically be charged to a particular d.c. potential, and then light from an original printed or typed paper is allowed to fall on the drum discharging those areas of the drum corresponding to the white spaces on the paper. As the drum continues to revolve, the surface is rotated to a point where toner is dusted onto the drum. The toner will attach to the drum only at those points that are charged, thus forming a black-on-white image on the drum. Finally, this image is transferred electrostatically from the drum to a sheet of paper and fused thereon in a fusing process.
To accomplish this process a variety of corotrons are required. A corotron is a wire extending the axial length of the drum. It is outside of the drum and is surrounded on one side by the drum semi-conductor surface and on the other three sides by a channel. A.C. voltage in excess of three thousand volts is applied to the wire producing a corona between the wire and the channel. The presence of this alternating corona field will discharge the semi-conductor drum surface to a nominal zero volts. This variety of corotron is used to discharge the semi-conductor drum surface prior to cleaning the excess toner from the drum after the image has been transferred to the paper.
Another variety of corotron is used to charge the semi-conductor drum surface, typically to 700 or 800 volts d.c., prior to exposing the image of the paper hard copy onto the drum.
Since each corotron requires a different d.c. component, it is typical that each corotron must have its own power supply. These power supplies typically comprise an a.c. source which is usually line voltage transformed to several thousand volts for application to the corotron in series with a regulated and adjustable d.c. power supply to supply the d.c. bias.
A problem with this variety of power supply is its high cost and relative lack of reliability since two separate power supplies must be provided, both of which contain regulation and adjustment circuits in their high-voltage lines. Furthermore, the cost of these power supplies is relatively high since a separate a.c. and d.c. power supply must be constructed, and these power supplies must have a large number of high-voltage parts.
A further problem is that the corotron is itself a rectifier. More specifically, a corotron will be created and conduction will begin when the corotron wire has an instantaneous potential of three thousand volts negative while a corona and conduction will not start until the central wire has about three thousand, two hundred volts of instantaneous potential in a positive direction. This is a result of the phenomenon that electrons will leave a sharp surface more readily than a flat surface.
It is because of this rectification by the corotron, and the resulting net d.c. bias on the corotron wire when high alternating voltages are applied that a d.c. series power supply must be supplied in a corotron power supply even when a net zero d.c. bias is required for the corotron's proper operation. It is therefore required of a corotron power supply that it has an adjustable d.c. series power supply even if a zero volt bias is required.
A further complication arises in the operation of a copier corotron since the amount of d.c. bias current required may vary depending on atmospheric humidity and pressure and paper thickness even while the a.c. current must remain constant. For this application, the a.c. current must be adjustable and regulated by a closed loop regulator circuit while the d.c. bias generating circuit must vary in accordance with operating conditions.
It can therefore be seen that there is a need in the copier industry for a corotron power supply which can simultaneously supply regulated and adjustable high-voltage a.c. current and variable d.c. current. It would be desirable for these two power supplies to be integrated into a single circuit; and it would reduce cost and increase reliability if the adjustment and regulation of these voltages could be done at a low-voltage level.